A typical direct current (DC) brushless motor drive may use a position sensing device, such as, a Hall effect or an optical sensor, to detect the instantaneous position of its rotor and thereby control the electronic switch to perform current direction change. However, the use of position sensors increases cost and reduces system reliability. Therefore, in recent years, various efforts have been made to remove the use of a position sensor in a brushless motor drive.
Therein, the back electromotive force (BEMF) signal is most commonly used in the sensorless motor technology. Since the BEMF varies according to the position and rotational speed of the rotor, the BEMF signal is used to determine the actual position of the rotor. However, the sensorless technology is mostly used for three-phase DC brushless motor drive. The main reason is that the three-phase drive motor is only turned on at two points at any single time, and therefore the other point can be used as BEMF signal measurement. Once the motor begins to rotate, the rotor position can be detected through the inductive BEMF on the stator winding. By processing these BEMF signals, in addition to determining the actual position of the rotor, it is also possible to control the switching of the excitation current of the respective stator winding coils for commutation. On the other hand, since there is no available effective BEMF signal measurement method for single-phase DC brushless motors, a suitable sensorless technology solution to solve the above-mentioned problems associated with the use of position sensors is yet proposed so far.